1. Field of the Invention
The present invention relates to a manganese oxide and process for preparing the oxide, a lithium manganese complex oxide using the manganese oxide and process for preparing the complex oxide, and further to a cobalt-coated lithium manganese complex oxide and process for preparing the cobalt-coated complex oxide. More specifically, the present invention relates to a manganese oxide giving a lithium manganese complex oxide, and a lithium manganese complex oxide using the manganese oxide and to a cobalt-coated lithium manganese complex oxide, which provide a particularly high discharge capacity and are useful for the improvement of cycle characteristics of a secondary battery as an active material of a positive electrode for a secondary battery with a nonaqueous electrolyte.
2. Description of the Prior Art
In recent years, there has been a rapid shift to portable and cordless types of electronic apparatus such as an AV device and a personal computer. Accordingly, there is an increasing demand for a secondary battery characterized by being small-sized and lightweight and having a high energy density. For the foregoing reason, a lithium-ion secondary battery is drawing attentions because this battery has a particularly high charge and discharge voltage and large charge and discharge capacity.
Heretofore, generally known as materials for a positive electrode useful in a high-energy lithium-ion secondary battery of a 4V grade are, for example, spinel-structured LiMn2O4 as well as rock salt-structured LiMnO2, LiCoO2, LiCo1xe2x88x92xNixO2, and LiNiO2. Among these substances, LiCoO2 is advantageous in terms of a high voltage and high capacity. However, LiCoO2 is disadvantageous in terms of high production cost due to the scanty supply of cobalt-containing raw materials and the problem associated with the safety of environments by waste batteries made of LiCoO2. Therefore, intensive studies are being made about a spinel-structured lithium manganese complex compound (LiMn2O4) which is produced from manganese available in copious amounts at a lower cost and environment-friendly.
A problem with a system using LiMn2O4 as a material for a positive electrode of a lithium-ion secondary battery, however, is that this system is inferior in charging discharging cycle characteristics despite a high voltage and a high energy density that are obtainable. Presumably, direct causes of the problem are that the deintercalation and intercalation of lithium ions in the crystal structure brought about by the repetition of charging and discharging operations expands and contracts the crystal lattice to destroy it by a change in crystal volume, and that Mn dissolves in the electrolyte.
The following three types of techniques are exemplified as main means employed in prior art for developing materials which prevents the charge discharge capacity from being degraded due to the repetition of charging and discharging operations to thereby improve charging discharging cycle characteristics.
(a) Homogenization of Composition in a Lithium Manganese Complex Oxide
The techniques of this type are, for example, those described in Japanese Patent Laid-Open Publication (Kokai) Nos. 9-86,933, 9-306,493, 9-129,233, 9-259,863, 10-3,194, 8-217,451, 9-147,859, 9-124,321, 10-21,914, 9-180,723, 9-306,490, 9-50,811, 10-83,816, 10-172,568, 10-162,826, 10-172,569, 10-501,369, 7-101,727, 8-315,823, 4-198,028, 7-97,216, 8-217,452, 6-295,724, 10-81,520, 10-81,521, 9-147,867, 10-130,024, 10-130,025, 9-147,859, 10-162,826, and 10-265,224.
(b) Stabilization of Skeletal Structure of a Base by the Addition of Elements
The techniques of this type are, for example, those described in Japanese Patent Laid-Open Publication (Kokai) Nos. 9-147,867, 9-134,723, 9-270,259, 9-213,333, 10-40,911, 10-3,918, 10-21,910, 10-172,568, 8-217,451, 8-217,452, 2-60,056, 10-241,682, 10-241,685, 10-241,686, 10-241,687, Japanese Patent Nos. 2,584,123, 2,584,246, and 2,627,314, A. D. Robertson et al., J. Electrochem. Soc., 144 (1997) 3500, A. D. Robertson et al., J. Electrochem. Soc., 144 (1997) 3505, J. M. Tarascon et al., J. Electrochem. Soc., 138 (1991) 2859, Japanese Patent Laid-Open Publication (Kokai) Nos. 9-259,863, 9-265, 984, 10-116,603, 10-188,953, 5-283,077, and 10-177,860.
(c) Inhibition of the Dissolution of Manganese by Surface Modification
The techniques of this type are, for example, those described in Japanese Patent Laid-Open Publication (Kokai) Nos. 10-3,194, 10-116,615, 10-199,528, WO97/23,918, and G. G. Amatucci et al., Solid State Ionics, 104(1997) 13.
In the case of type (a) described above, Japanese Patent Laid-Open Publication (Kokai) Nos. 9-86,933, 4-198,028 and 7-97,216 report an attempt to adjust the kind, shape, and size of precursors, while Japanese Patent Laid-Open Publication (Kokai) No. 6-295,724 reports an attempt to enhance the reactivity with lithium by employing mechanical pulverization and classification from the standpoint of carrying out homogeneous blending. On the other hand, Japanese Patent Laid-Open Publication (Kokai) No. 9-147,859 discloses the preparation of LiMn2O4 by using a technique such as a sol-gel process or spray drying. However, none of these techniques are satisfactory because of problems such as technical limitations and industrial problems.
In the case of type (b) described above, an element having a valency of 3 or less is found to be effective in the improvement of cycle characteristics. This technique, however, cannot be a substantial solution because a remarkable reduction in battery capacity is unavoidable due to the decrease of the amount of Mn3+ which determines the battery capacity as an important characteristic of a secondary battery.
Furthermore, in the case of type (c) described above, for example, WO97/23,918 discloses a process comprising coating a manganese compound as a precursor of a lithium manganese complex oxide or a lithium manganese complex oxide with a non-manganese metal element, blending the coated product with a lithium salt, thereafter, sintering the resultant blend. Because of this process, it is difficult to consider that the non-manganese metal element sufficiently covers the surface of the lithium manganese complex oxide. In addition, since the non-manganese metal element on the surface also becomes a lithium compound, substances which have different electrochemical characteristics may be formed at the time of charging and discharging operations. Consequently, the battery characteristics are liable to become poor.
It is an object of the present invention to provide a secondary battery with a non-aqueous electrolyte, which is characterized by the realization of a high-level balance between charge-discharge capacity and cycle characteristics without using techniques of prior arts such as substitution of Mn or Li with other elements or surface treatment, by using as the active material of a positive electrode a spinel-structured lithium manganese oxide obtained by preparing a manganese oxide having a very good particle size distribution and using the obtained manganese oxide as the precursor.
Another object of the present invention is to provide a cobalt-coated lithium manganese complex oxide useful as the active material of a positive electrode for a secondary battery with a non-aqueous electrolyte, which secondary battery is characterized by the realization of a high-level balance between charge-discharge capacity and cycle characteristics, by preparing the cobalt-coated lithium manganese complex oxide by epitaxially growing a cobalt oxide on the surface of a lithium manganese oxide so that a predetermined-coating amount of cobalt is obtained.
Still further objects of the present invention will become apparent to those skilled in the art from the following detailed description.